This invention relates to an apparatus and method for the electrodeposition of aluminum and its alloys onto surfaces of electrically conductive materials, using an aprotic, oxygen-free, water-free organoaluminum electrolyte, and more specifically to an improved apparatus and method of this nature for pretreating and then coating tumbling work pieces of any design in a controlled air-tight and water-free environment.
Reactive metals such as aluminum are electrodeposited from many aprotic, oxygen-free organoaluminum electrolytes. Removal of all surface oxides and water from the basis metal before and during electroplating greatly facilitates adhesion of the aluminum deposit.
Early attempts to deposit aluminum using open containers were inefficient because the organoaluminum electrolytes react rapidly with oxygen and moisture in the air, causing the electrolyte to lose conductivity and useful life. Therefore, most current state of the art aluminum electrodeposition is conducted in liquid and gas-tight chambers to enhance the life of the electrolyte. Some provide airlock chambers between the electroplating chamber and the solvent inlet and outlet means to avoid or reduce the introduction of air and water into the electroplating chamber during the filling and draining steps. These systems generally provide for the placing of pretreated, water-free workpieces into a chamber with a water and air-free inert solvent, replacing the solvent with inert gas, and then filling the plating chamber with electrolyte and electroplating. Examples of such systems are shown in U.S. Pat. Nos. 4,066,515 and 4,392,936.
The main disadvantages in the prior art are the difficulty in pretreating workpieces so as to render them free from water and air, the large dragout volumes of the various treatment liquids (the volume retained of the drainage due to adherence to the barrel and the workpieces), the difficulty of operator access to the plating cell (for the charging and emptying of cathode work pieces and for routine maintenance such as replacing aluminum anode material). Another disadvantage, which is important but not immediately obvious, results from the use of the relatively low conductance, high capacitance solutions associated with non-aqueous aluminum plating electrolytes. Generally, the ratio of the anode to cathode surface area is relatively low, and the cathode to anode spacing is relatively large. Thus, relatively high voltages have been needed to accomplish the electrodeposition, causing increased electrical consumption, decreased maximum attainable current density, and loss of plating efficiency.